This invention relates generally to photometers, radiometers, and light sources, and specifically to (1) a radiometer whose response varies with the angle of incident radiation according to virtually any desired function of the incident angle and (2) a light source whose emission intensity varies according to virtually any desired function of the emission angle.
In a variety of scientific and technological endeavors, an angle-dependent measure of the optical intensity at a point in space is desirable. Most familiar of possible angular responses is the Lambertian (or cosine) response for which the response of the radiometer is proportional to the cosine of the angle of incidence (with normal equal to zero degrees). The ultraviolet exposure response of photoresist during semiconductor device fabrication, for example, is Lambertian. A radiometer with Lambertian angular response and a spectral response appropriate to the particular photoresist would be a valuable tool for the calculation of photoresist exposure times. Another angular response of value is one that responds equally in all angles of incidence. This would be of use in the measurement of laser radiation where the alignment of radiometer to laser beam is impractical (as when the beam is invisible) or when measuring intensity from a source of unknown direction.
While many currently available radiometers are Lambertian in response, none remains so after reducing the numerical aperture by the addition of optical filters, apertures, and/or a detecting surface remote from the light-gathering area. These numerical aperture decreasing techniques result in a radiometer that is clearly non-Lambertian, if only by virtue of its lack of response to light rays outside of the reduced acceptance angle.
Heretofore, angular response correcting devices for light measurement instruments were large and relatively expensive, such as integrating spheres (e.g., U.S. Pat. No. 4,310,246 to Blazek, Jan. 12, 1982); the multiple detector device of U.S. Pat. No. 4,420,252 to Nakauchi, Dec. 13, 1983; or the complex array of spheres of U.S. Pat. No. 4,511,250 to Olsen, Apr. 16, 1985. Some are suited only for shaping the angular response of one particular instrument to only one corrected response (e.g., U.S. Pat. No. 4,391,521 to Imai et al., July 5, 1983).
None of the prior art patents provides the versatility of the present invention in its ability to correct nearly any angular response to the shape of nearly any other angular response. Furthermore, the present invention is very inexpensive to fabricate, can acquire light in the tiniest of spaces, and is well-suited to the measurement of ultra-high intensity sources, such as those used in wafer step-and-repeat machines in the semiconductor photolithography field. This high intensity suitability arises from the relative insensitivity of the fiber optic element to the heat generated by high-intensity light and the capability for placing the temperature-perturbable photodetective element at the far end of the fiber optic, safely away from heat.
Finally, the present invention allows for the insertion of any number of spectral filters to shape the spectral response of the radiometer without sacrificing angular response; the correcting element is simply designed to reshape the angular response to accommodate the filters.
The essential elements of the present invention--the fiber optic and correcting element--can be juxtaposed to provide another feature, the shaping of the angular emission of a light source. The resultant light source of shaped angular emission would be desir able for navigational aids and beacons, traffic signals with selective lane control, guidance systems, displays, or in reshaping the beam intensity profile of a collimated source as illustrated in FIG. 5 and described later.